1. Field of the Invention
The present invention is directed to an improved system for lyophilizing with microwaves and an improved method for microwave lyophilization.
2. Prior Art
Lyophilization, or freeze drying, as it is more commonly known, is used in a number of different industries to remove water from materials to achieve a more stable pure product with a prolonged shelf life. The process is used in the pharmaceutical and food industries which require lyophilization systems that are capable of producing environmental processing conditions to effect sublimation so that the water is removed from processed materials. The water vapor is drawn off from the lyophilization chamber and typically removed by trapping on a refrigerated condenser surface, desiccants or other suitable devices.
Sublimation is a process wherein materials change from a solid phase directly to a gaseous phase without passing through a liquid phase. With water, ice turns directly to water vapor without first melting to a liquid form, and then evaporating. Sublimation can occur at various temperatures and pressure combinations, but typically sublimation needs low temperatures and a vacuum pressure less than atmospheric. Sublimation provides advantages for materials processing as purity is maintained and the processed material does not have to be subjected to high temperatures, such as would be needed to boil off the water.
Although traditional lyophilization systems have worked well for their intended purpose, they have several shortcomings. Traditional lyophilization systems must attain subzero temperatures and create vacuum conditions to provide atmospheric processing conditions that facilitate sublimation. These types of lyophilization systems have shortcomings that lessen their usefulness. Such systems require large amounts of energy for refrigeration equipment, for creating and maintaining the vacuum, and for providing the heat, primarily through convection and conduction, for sublimating the ice and warming the product and the system. In addition, to compound the high energy consumption, such traditional lyophilization processes are very time consuming. Often, the freeze drying may take a week or more, creating a bottleneck in the material processing. To accommodate high production needs, the size of the freeze drying systems must be quite large to handle large batches. Furthermore, should problems develop during the freeze drying process, large batches of material may be damaged. As the systems require large amounts of energy to maintain the atmospheric conditions for an extended period of time, the operating costs are high, thereby increasing the total cost of processing the product.
To increase the speed of the drying process and to decrease the amount of energy required for heating, including energy necessary to heat the mass of shelving for radiation, convection and conductive heating of the material to be processed, systems and methods have been developed that use microwaves to aid freeze drying. Although for freeze drying, such systems still require vacuum and a condenser or other system for collecting the liberated water vapor, the energy needed to maintain temperatures for sublimation is decreased as microwaves are used in the sublimation process. Such systems achieve freeze drying of the materials, but do so in greatly reduced time periods. Processing taking several days or a week or more with conventional lyophilization may now be performed in less than a day, and in many cases, several hours. The microwaves provide the energy of sublimation directly to the materials being processed, alone or in combination with radiation, convection and/or conduction, so that sublimation occurs much more efficiently.
Though microwaves have been used to speed the freeze drying process, and are successful when operated and controlled correctly, there are problems associated with such systems. Prior microwave systems operating under vacuum conditions suffer from uncontrollable corona discharge, which occurs when high electric fields ionize gases within the freeze drying chamber. Sharp edges of metallic objects can enhance the local electric field and ignite gases and create a corona discharge. Such occurrences of corona discharge create localized temperature spikes that may cause localized overheating or melting, adversely affecting the materials near the occurrence. This affects the quality of the freeze dried product, since many products, including many pharmaceutical and biological products are temperature sensitive, have very high quality standards. Corona discharge can be fatal to the success of the freeze drying process. Non-uniform microwave coverage can also adversely affect the quality of the product being processed.
Heretofore, prior art microwave systems have not employed a method of successfully reducing or eliminating corona discharge within the freeze drying chamber. Moreover, such systems have not employed detectors to sense when corona discharges occur. Even if they had detected problems, such systems do not have controls to adjust conditions in response to detected arcing in order to minimize or eliminate the occurrences of corona discharge in time to reduce damage to the product.
Examples of freeze drying apparatuses using microwaves to assist in drying are shown in U.S. Pat. Nos. 2,859,534 and 3,020,645 to Copson, and U.S. Pat. NO. 3,048,928 to Copson et al. Although the Copson patents teach microwave friendly trays to limit discharge in the processing chamber, and removing condensation coils from the inner processing container, no additional steps are shown or suggested to actively control and monitor microwave discharge. U.S. Pat. No. 3,264,747 to Fuentevilla teaches a microwave assisted freeze drying apparatus using non-conductive materials such as Plexiglas to contain the product. Although microwaves are utilized, there is no system for detection, control, and/or elimination of corona discharge.
A major hurdle with detection systems is that temperature sensors typically are made of materials that, if extended into the microwave field, would create further discharges. Therefore, traditional temperature, pressure, and other sensors to be placed within the microwave field often cannot be utilized without modification.
It can be seen then that a need exists for a new and improved system for microwave assisted lyophilization. Such a system should greatly reduce the time and energy required to uniformly freeze dry the material being processed. In addition, such a system utilizing microwave energy should be configured to minimize the potential effects of corona discharge within the lyophilization chamber. The system should provide microwave distribution to all materials placed in the chamber and provide relatively uniform processing of the materials in the chamber. Such a lyophilization system should also utilize detectors and controls to detect the occurrence of actual and/or incipient corona discharges and to adjust the microwave field strength and other system characteristics to promptly eliminate corona discharges when detected. The present invention addresses these as well as other problems associated with microwave lyophilization systems.